DC-DC converter and device for operation of a high pressure discharge lamp using said converter

ABSTRACT

A DC-DC converter which accomplishes a reduction of the switching loss in the wide, variable range of the continuity ratio of the main switching device includes a direct current source, an ON-OFF-controllable main switching device, a main coil which is series connected to the main switching device, a fly-wheel diode which is arranged such that the induction current of the above described main coil flows when the main switching device is shifted into the OFF state, a smoothing capacitor for smoothing the output of the main coil, an auxiliary coil, a resonant capacitor and an ON-OFF-controllable auxiliary switching device, in which the auxiliary coil and the resonant capacitor are series-connected to form a LC series connection. Further, the LC series connection, the main switching device and the fly-wheel diode are also series connected, with the auxiliary switching device being connected in parallel to the LC series connection.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a DC-DC converter of the voltage reduction-bucktype with the PWM (pulse width modulation) method in which theefficiency is increased, and a device for using a DC-DC converter tooperate a high pressure discharge lamp such as a metal halide lamp,mercury lamp or the like.

2. Description of the Related Art

Conventionally, of the converters that convert the voltage of a DCsource into another value, output it and supply it to a load, i.e. DC-DCconverters, the converter of the voltage reduction-buck type, which isshown in FIG. 15, is often used to carry out voltagereduction-conversion.

In this circuit, the current from the DC source (Vin) is repeatedlyshifted by a main switching device (Qx′) such as a FET or the like intothe ON state or the OFF state, and a smoothing capacitor (Cx′) ischarged via the main coil (Lx′). In this arrangement, this voltage canbe applied to a load (Zx).

During the interval in which the above described main switching device(Qx′) is in the ON state, charging of the smoothing capacitor (Cx′) andcurrent supply to the load (Zx) are carried out directly by the currentthrough the main switching device (Qx′), and moreover in the main coil(Lx′), energy is stored in the form of a flux. During the interval inwhich the main switching device (Qx′) is in the OFF state, the smoothingcapacitor (Cx′) is charged via a fly-wheel diode (Dx′) by the energystored in the form of a flux in the main coil (Lx′) and current issupplied to the load (Zx).

This converter is operated under PWM control of the main switchingdevice (Qx′). Specifically, by feedback control of the ratio between thetime interval in the ON state and the sum of the period of the ON stateand the period of the OFF state of the main switching device (Qx′), i.e.the continuity ratio, the voltage supplied to the load (Zx) can beadjusted, even as the voltage of the DC source (Vin) fluctuates, to adesired (for example, constant) value, the supplied current can beadjusted to a desired value and the supplied wattage can be adjusted tothe desired value.

Of course, the value of the desired efficiency (voltage, current,wattage or the like) can assume a constant value or it can also bechanged over time. For feedback control of the desired efficiency, adetector is needed to determine the output voltage and the outputcurrent, as is a feedback control circuit, which is not shown in thedrawings.

FIG. 16 shows the voltage and current waveform of this converter usingone example. If the main switching device (Qx′) is shifted into the ONstate, the voltage (VxD′) supplied to the main switching device (Qx′)passes from the voltage of the DC source (Vin) essentially to 0 V.However, this transition does not take place instantaneously, butrequires a certain time.

Here, in the process in which the voltage (VxD′) of the main switchingdevice (Qx′) gradually decreases, the current (IQx′) of the mainswitching device (Qx′) also gradually begins to flow. There is thereforean interval during which neither the voltage (VxD′) nor the current(IQx′) is 0. According to the size of the time integral of the productof the voltage and the current, for each transition of the mainswitching device (Qx′) into the ON state a switching loss (SwL) occurson the main switching device (Qx′).

This switching loss also arises by the same process in the case of thetransition into the ON state as in a transition into the OFF state. Butnormally the loss in the transition into the ON state is greater. Thereason is that, when the main switching device (Qx′) is a FET, forexample, a parasitic electrostatic capacitance is present between thesource electrode and the drain, that the electrical charge which hasbeen charged onto this electrostatic capacitance during the interval ofthe OFF state of the main switching device (Qx′) at the voltage of theDC source (Vin), in the transition into the ON state is subjected toforced short circuit discharge, and that the energy which is consumed indoing so is added to the switching loss (SwL).

When this switching loss is present, there is not only the disadvantageof a reduction in the efficiency of the converter, but also thedisadvantage of a large converter and a cost increase of it, since theheat generation of the main switching device (Qx′) is large and sincetherefore a switching device with large maximum power dissipation mustbe used and there must be a large radiator with high radiationefficiency in addition. Furthermore, the fan that supplies cooling airfor cooling the radiator must be a high capacity fan, which brings thedisadvantages of the reduction in the efficiency and the increase insize and cost of the converter.

In order to eliminate these disadvantages, conventionally a host ofproposals have been made. They are mainly technologies that preventintervals during which neither the voltage (VxD′) nor the current (IQx′)is 0. Normally the technology in which switching is carried out at a 0voltage of the switching device, is called zero voltage switching, andthe technology in which switching is carried out at a 0 current of theswitching device, is called zero current switching. Often, using aso-called LC resonance the voltage applied to the switching device andthe current flowing in the switching device are temporarily taken overby the voltage induced by the L component (coil) and the current flowingin the C component (capacitor) and are essentially set or reduced to 0,and during this time a transition of the switching device into the ONstate or the OFF state is carried out.

For example, in Japanese patent disclosure document HEI 1-218352 a DC-DCconverter of the voltage reduction-buck type with current resonance isproposed. In this proposal the current flowing in the main switchingdevice (Qx′), however, due to resonance has a higher peak value than aconventional DC-DC converter of the voltage reduction-buck type.Therefore, it becomes necessary to use a switching device with a highcurrent. Furthermore, in the case in which the switching frequency ishigher than the resonant frequency, it is possible that the losscontinues to increase because the switching device is shifted into theOFF state at a high current.

Additionally, in this circuit arrangement, according to the assumptionof a constant output voltage for a DC-DC converter, the PWM method isundertaken with a constant switching frequency. Because of this, it isnecessary to match the continuity ratio thereof to the resonantfrequency. The range of the continuity ratio is therefore limited. Anincrease of the efficiency can therefore only be accomplished in thevicinity of the rated output voltage. Neither a guideline nor conditionsfor a measure against the fluctuation of the load were considered.

Furthermore, for example, U.S. Pat. No. 5,880,940 discloses a DC-DCconverter of the voltage reduction-buck type in which a secondarywinding is added to the main coil (Lx′), and thus, a transformer isformed.

In this proposal, a DC-DC converter is described as being operated byconnecting an auxiliary switching device to the transformer as a forwardconverter. However, an increase of the ripple in the output current bythis operation was not even considered. The added auxiliary switchingdevice cannot be subjected to zero voltage switching either. It isnecessary to add another coil and to carry out zero current switching.

In the case of zero current switching, different from zero voltageswitching, there is specifically the disadvantage that the problem ofpower consumption loss as a result of the forced short circuit dischargeis not eliminated in the transition of the electrical charge into an ONstate which was charged in the parasitic electrostatic capacitance ofthe main switching device. Therefore, this is not ideal.

On the other hand, if the use of a DC-DC converter of the voltagereduction-buck type is considered, the resonant conditions of the LCresonance circuit are easily satisfied in a stable manner, since theoutput voltage is relatively stable for applications such as a constantvoltage current source or the like.

However, in the case of use as a device for operating a high pressuredischarge lamp, such as a metal halide lamp, a mercury lamp or the like,the lamp voltage as the output voltage is changed significantly by thestate of the lamp as a load. Under certain circumstances it fluctuatessteeply. Therefore, a specially adapted construction is needed. Theconverter must also be matched to this construction.

The feature of the high pressure discharge lamp as the load of theconverter is described below. Generally a high pressure discharge lamp(Ld) has an arrangement in which a discharge space (Sd) is filled with adischarge medium which contains mercury and in which a pair ofelectrodes (E1, E2) is located oppositely for the main discharge.Between the electrodes (E1, E2) an arc discharge is produced and theradiation emitted from the arc plasmas is used as the light source.

The high pressure discharge lamp (Ld), different from a general load,exhibits a property which is closer to a Zener diode than to animpedance element. This means that the lamp voltage does not changegreatly, even if the flowing current changes. A lamp voltage whichcorresponds to a Zener voltage however changes greatly depending on thedischarge state.

Specifically, in the state before the start of the discharge, the Zenervoltage is extremely high because no current at all is flowing. If, byoperating a starter, such as a high voltage pulse generator or the like,a discharge is started, a glow discharge is formed. In the case, forexample, of a discharge lamp which contains greater than or equal to0.15 mg of mercury per cubic millimeter of volume of the discharge space(Sd), the glow discharge voltage ranges from 180 V to 250 V. In thestate before the start of the discharge, a voltage of at least equal tothe glow discharge voltage is applied to the high pressure dischargelamp. Normally, this voltage is roughly 270 V to 350 V and is called theno-load voltage. The starter is operated in this way.

When the electrodes (E1, E2) are heated by the glow discharge to asufficient degree, a sudden transition into an arc discharge takesplace. Immediately after the transition a low arc discharge voltage from8 V to 15 V is shown. This is a transient arc discharge. The arcdischarge vaporizes the mercury and if heating of the mercury vaporcontinues, the arc discharge voltage gradually increases until itreaches a steady-state arc discharge from 50 V to 150 V. The voltage ina steady-state arc discharge, i.e., the lamp voltage, depends on thedensity of the mercury which has been added to the discharge space (Sd)and the distance between the electrodes (E1, E2).

Immediately after the transition into the arc discharge, depending onthe vapor state of the mercury, the glow discharge suddenly returns orthe arc discharge and the glow discharge takes place alternately in avigorous back and forth manner.

At a constant voltage from the DC source (Vin) the output voltage of theDC-DC converter of the voltage reduction-buck type is at a value whichis obtained by multiplying roughly the voltage of the DC source (Vin) bythe continuity ratio. Therefore, the DC-DC converter of the voltagereduction-buck type can be kept approximately for the DC-constantvoltage current source.

On the other hand, in idealized switching theory, in the case in which aDC-constant voltage current source is connected to a Zener diode as aload, i.e., still another DC-constant voltage current source, the theoryfails and good analysis is not possible. More accurately, when in thecase of connecting a Zener diode as the load to a constant voltagecurrent source, the output voltage of the constant voltage currentsource is lower than the Zener voltage, no current at all flows in theZener diode. Conversely, in the case in which the output voltage of theconstant voltage current source is higher than the Zener voltage, aninfinitely large current flows.

In the case in which a discharge lamp which can be roughly regarded as aZener diode is connected to a realistically present DC-DC converter ofthe voltage reduction-buck type as a load, extinction of the dischargeoccurs in the case in which the output voltage of the converter is lowerthan the Zener voltage. Conversely, in the case in which the outputvoltage of the converter is higher than the Zener voltage, an undulyhigh current which is determined by the current serviceability of the DCsource (Vin) and of the converter flows in the lamp.

Therefore, in a device for operating a high pressure discharge lamp, thefollowing is required of a converter for supplying a high pressuredischarge lamp:

There is a demand for the property which enables a prompt change of thecontinuity ratio in a wide, variable range for PWM control according tothe discharge voltage of the high pressure discharge lamp in order toprevent extinction of the discharge from occurring or an unduly largecurrent from flowing and the lamp and converter circuit from beingdamaged. These must be achieved even at a discharge voltage thatcorresponds to the no-load voltage which changes in this way to a greatextent and also vigorously depends on the discharge state, i.e., thestate in which a no-load voltage is applied (state before the start ofdischarge), the glow discharge state, the state of a transient arcdischarge, or the steady-state arc discharge state. Furthermore there isa demand for a property that enables maintenance of operation in whichthe switching loss is reduced by resonant operation.

In the case of high ripple which is contained in the current flowing inthe discharge lamp, there is a case in which instability, flicker andextinction of the discharge arise due to acoustic resonance. Therefore,it is required of the converter that the ripple of the output current issmall. As a result, it is necessary to prevent the operation of theresonant circuit which is arranged for reducing switching loss fromaccelerating the formation of a superfluous ripple component.

In the case, for example, of a DC-DC converter of the voltagereduction-buck type which is described in the above cited U.S. Pat. No.5,880,940, the main coil also acts as a transformer with a resonantoscillation effect. Originally during the interval in which the mainswitching device is in the ON state, in base operation of the DC-DCconverter of the voltage reduction-buck type on its two ends the maincoil has a difference voltage between the supplied DC source voltage andthe output voltage and works in such a way that the input DC sourcevoltage is not applied directly to the load.

In the case of a great fluctuation of the output voltage, of course, thevoltage on the primary side of the transformer fluctuates greatly with aresonant oscillation effect. Since the energy transmitted to thesecondary circuit of the transformer also fluctuates greatly because ofthe resonant oscillation effect, as a result the resonant operation alsofluctuates greatly. The DC-DC converter of the voltage reduction-bucktype described in U.S. Pat. No. 5,880,940 is therefore not suited as aconverter for supplying a high pressure discharge lamp.

As was mentioned above, it is necessary in a DC-DC converter of thevoltage reduction-buck type to reduce the switching loss in order toavoid raising the size and costs of the converter. But in the prior artit was difficult to have a wide, variable range of output voltage andkeep down the cost because of the addition of the resonant circuit. Inparticular it was difficult to obtain a converter that is suited tooperate a high pressure discharge lamp.

SUMMARY OF THE INVENTION

A primary object of the present invention is to devise a DC-DC converterthat eliminates the disadvantage of a conventional DC-DC converter,i.e., the disadvantage of difficult implementation of a reduction of theswitching loss in a wide, variable range of the continuity ratio of themain switching device with low costs.

Another object of the invention is to devise a device for operating ahigh pressure discharge lamp that eliminates the disadvantage of aconventional device for operating a high pressure discharge lamp, i.e.,the disadvantage of difficult implementation of a reduction of theswitching loss with low costs.

According to a first aspect of the invention, for a DC-DC converter ofthe voltage reduction-buck type which comprises the following:

-   -   a direct current source (Vin);    -   an ON-OFF-controllable main switching device (Qx);    -   a main coil (Lx) which is series connected to the main switching        device (Qx);    -   a fly-wheel diode (Dx) which is arranged such that the induction        current of the above described main coil (Lx) flows when the        main switching device (Qx) is shifted into the OFF state; and    -   a smoothing capacitor (Cx) for smoothing the output of the main        coil (Lx), the object is achieved in that furthermore there are        an auxiliary coil (Lw), a resonant capacitor (Cw) and an        ON-OFF-controllable auxiliary switching device (Qw), that the        auxiliary coil (Lw) and the resonant capacitor (Cw) are        series-connected and thus form a LC series connection, that this        LC series connection, the main switching device (Qx) and the        fly-wheel diode (Dx) are series connected, that the auxiliary        switching device (Qw) is connected in parallel to the LC series        connection, and that the main switching device (Qx) and the        auxiliary switching device (Qw) are controlled such that they        are shifted in alternation into the ON state and moreover the        main switching device (Qx), after the auxiliary switching device        (Qw) has been shifted into the OFF state, is shifted into the ON        state within a given time.

According to one development of the above described invention, theseobjects are achieved by the following operations: in the state in whichthe resonance prohibition signal (Sc) is activated, the auxiliaryswitching device (Qw) is shifted into the ON state when the ON state ofthe main switching device (Qx) is reached, instead of being shifted intothe ON state alternately with the main switching device (Qx); and themain switching device (Qx) is shifted into the ON state within a giventime after the auxiliary switching device (Qw) has been shifted into theOFF state.

According to a second aspect of the invention, in a device for operatinga high pressure discharge lamp (Ld) in which the discharge space (Sd) isfilled with a discharge medium and there is a pair of opposed electrodes(E1, E2) for the main discharge, the objects are achieved in that theDC-DC converter described above is used for supplying the high pressuredischarge lamp (Ld).

Advantages

First of all, the action of the invention is described in its firstaspect.

In this invention, by the arrangement of the DC-DC converter which isdescribed in achieving the object, the auxiliary switching device (Qw)is shifted into the OFF state before the main switching device (Qx) isshifted into the ON state, furthermore in the auxiliary coil (Lw) avoltage is induced in the direction in which the main switching device(Qx) is biased in the backward direction, and the electrical charge ofthe parasitic electrostatic capacitance of the main switching device(Qx) is discharged via a fly-wheel diode (Dx). In this way, theinvention acts such that zero voltage switching is obtained when themain switching device (Qx) is shifted into the ON state. Details aregiven below.

Furthermore, as is described below, because control is exercised suchthat the auxiliary switching device (Qw) is shifted into the ON statewithin a given time after the main switching device (Qx) has beenshifted into the OFF state, the following can be achieved.

Before the auxiliary switching device (Qw) is shifted into the ON state,the main switching device (Qx) is shifted into the OFF state, in theauxiliary coil (Lw) the voltage is induced in the direction in which theauxiliary switching device (Qw) is biased in the backward direction andthe electrical charge of the parasitic electrostatic capacitance of theauxiliary switching device (Qw) is discharged. In this way zero voltageswitching can be obtained when the auxiliary switching device (Qw) isshifted into the ON state.

The invention is further described below using several embodiments whichare shown in the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of the circuit arrangement of a DC-DC converteraccording to the first aspect of the invention;

FIG. 2 shows a schematic of the voltages and the current waveforms whichcorrespond to the circuit arrangement of the DC-DC converter accordingto the first aspect of the invention;

FIG. 3 shows a schematic of the timing of driving of the main switchingdevice and the auxiliary switching device according to a development ofthe first aspect of the invention;

FIG. 4 shows a schematic of the timing of driving of the main switchingdevice and the auxiliary switching device according to a development ofthe first aspect of the invention;

FIG. 5 shows a schematic of the circuit arrangement of a DC-DC converteraccording to the second aspect of the invention;

FIG. 6 shows a schematic of another embodiment of the inventionaccording to the first aspect of the invention;

FIG. 7 shows a schematic of another embodiment of the inventionaccording to the first aspect of the invention;

FIG. 8 shows a schematic of the arrangement of a driver control element(Gw) and a feedback control element (Fb) of the DC-DC converter of theinvention;

FIG. 9 shows a schematic of one embodiment of the circuit arrangement ofthe driver control element (Gw) and of part of the feedback controlelement (Fb) of a DC-DC converter of the invention;

FIG. 10 shows a schematic of one embodiment according to the secondaspect of the invention;

FIG. 11 shows a schematic of another embodiment according to the secondaspect of the invention;

FIG. 12 shows a schematic of one embodiment of the control sequence ofthe third aspect of the invention;

FIG. 13 shows a schematic of one embodiment according to the thirdaspect of the invention;

FIG. 14 shows a schematic of the voltages measured in reality and ofcurrent waveforms of the DC-DC converter according to the third aspectof the invention;

FIG. 15 shows a schematic of the circuit arrangement of a conventionalDC-DC converter of the voltage reduction-buck type; and

FIG. 16 shows a schematic of the voltages and of the current waveformsof the circuit arrangement of a conventional DC-DC converter of thevoltage reduction type.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the arrangement of the circuit of a DC-DC converter of theinvention in a simplified representation. FIG. 2 shows essentially therespective waveform in the circuit shown in FIG. 1.

This circuit has the same arrangement as in the conventional DC-DCconverter of the voltage reduction-buck type, in which the followingtakes place:

-   -   during the interval in which the main switching device (Qx)        which consists of a FET or the like is in the ON state, a        current flows from the DC source (Vin) via the main coil (Lx)        which is series connected to the main switching device (Qx).        Here furthermore the smoothing capacitor (Cx) of the main coil        (Lx) which is connected to the terminal opposite the main        switching device (Qx) is charged, current is supplied to the        load (Zx) which is connected in parallel to the smoothing        capacitor (Cx) and moreover energy in the form of a flux is        stored in the main coil (Lx). During the interval in which the        main switching device (Qx) is in the OFF state, the smoothing        capacitor (Cx) is charged by the energy stored in the main coil        (Lx) in the form of a flux via the fly-wheel diode (Dx) in which        a cathode is connected to one node between the main switching        device (Qx) and the main coil (Lc) and current is supplied to        the load (Zx).

In this circuit arrangement, in addition to the same arrangement as thearrangement of the conventional DC-DC converter of the voltagereduction-buck type, the following actions are implemented:

-   -   the auxiliary coil (Lw) and the resonant capacitor (Cw) are        series connected and thus a LC series connection is formed;    -   This LC series connection is connected such that the DC source        (Vin), the main switching device (Qx) and the fly-wheel diode        (Dx) are series connected; and    -   The auxiliary switching device (Qw) is connected in parallel to        the LC series connection.

Here, the basic principle is that the main switching device (Qx) and theauxiliary switching device (Qw) are operated such that one of the two isshifted into the OFF state when the other is in the ON state. However,control is exercised such that the auxiliary switching device (Qw) isshifted beforehand into the OFF state by a switch closing prohibitioninterval (τy), which is described below, before the main switchingdevice (Qx) is shifted into the ON state.

During the interval shown in FIG. 2, from a time (t1) until a time (t2)is reached, the main switching device (Qx) is in the ON state. However,the auxiliary switching device (Qw) is in the OFF state. Current supplyfrom the DC source (Vin) to the load side is therefore carried out viathe auxiliary coil (Lw) and the resonant capacitor (Cw). This means thatcurrent supply is carried out as the resonant current of the LC seriesconnection. The resonant phenomenon here is a LC resonance by theinductance of the auxiliary coil (Lw) and the resonant capacitor (Cw).

During the interval from the time (t1) until the time (t2) is reached,the resonant capacitor (Cw) is charged, resulting in a voltage that isgradually reduced and applied to one terminal on the input side of themain coil (Lx). To prevent this amount of reduction of the voltage frombecoming too large by the time the interval (Ton) ends and the capacityfor current feed to the switching parts starting with the main coil (Lx)from being additionally reduced, it is advantageous for the resonantcapacitor (Cw) to have electrostatic capacitance in a sufficient amount,considering the switching frequency of this circuit.

During this interval, the resonant capacitor is charged and electricalenergy is stored. In doing so the current flows in the auxiliary coil(Lw) and magnetic energy is stored in it. This means that resonantenergy is stored in the LC series connection. This energy is consumed inorder to later carry out resonant operation.

If next, at the time (t2), the main switching device (Qx) is shiftedinto the OFF state, as is described below, the voltage between theterminals of the auxiliary switching device (Qw) passes into a lowstate, so that the voltage of the DC source (Vin) is applied to the mainswitching device (Qx). Therefore, in the parasitic electrostaticcapacitance of the main switching device (Qx), the electric charge ischarged up to this voltage.

Since at the same time (t2) energy is stored in the LC seriesconnection, in the closed loop comprised of the auxiliary coil (Lw), theresonant capacitor (Cw) and the auxiliary switching device (Qw), theresonant current flows without interruption. With respect to theauxiliary switching device (Qw), the current begins to flow, however,via an antiparallel diode (Dqw) which is connected in parallel thereto.

The antiparallel diode (Dqw) is present as an outside element, forexample, in the case in which the auxiliary switching device (Qw) is aMOSFET. It can also be used as such.

With respect to the timing for turning on the auxiliary switching device(Qw), it is advantageous to shift as quickly as possible the auxiliaryswitching device (Qw) into the ON state while ensuring enough time toprevent this timing from coinciding with the ON interval of the mainswitching device (Qx) before the main switching device (Qx) is shiftedinto the OFF state. The reason for this is that during the interval inwhich current is flowing in the antiparallel diode (Dqw), a forwardvoltage of the antiparallel diode (Dqw) forms and that if in doing sothe auxiliary switching device (Qw) is in the ON state, the forwardvoltage of the antiparallel diode (Dqw) can be reduced. By the sameprinciple as in the case of a so-called synchronous rectification theloss in the above described antiparallel diode (Dqw) and in theauxiliary switching device (Qw) can be reduced. This is one of theadvantages of the invention.

Since the peak value of the resonant voltage applied to the resonancecapacitor (Cw) changes with the different constants of the componentscomprising the circuit, combinations of different constants can be usedin conjunction with the maximum ratings of the components used and thecosts are advantageous.

The peak value of the voltage applied to the resonant capacitor (Cw) isessentially proportional to the output wattage of the DC-DC converter ofthe voltage reduction-buck type. For example, for constant powerregulation the peak value of the voltage applied to the resonantcapacitor (Cw) is essentially constant. In the case of a small outputvoltage the peak value of the voltage applied to the resonant capacitor(Cw) is reduced, which raises the possibility that resonant operationdoes not take place to a sufficient degree. But since the output wattageis small, and since thus originally the switching loss is also small, ofthe invention, this is not regarded as disadvantageous. Therefore, tocarry out resonant operation under the condition which is similar to themaximum utilization output wattage, the different constants of thecomponents comprising the circuit can be adjusted.

In the circuit arrangement of the invention, by the measure that thereare an auxiliary coil (Lw) and a resonant capacitor (Cw) which areindependent of the basic (conventional) DC-DC converter part of thevoltage reduction-buck type, and that resonant operation is carried out,a reduction of the switching loss is desired. Therefore the differentconstants of the switching devices comprising the resonant circuit,i.e., the parameters of the resonant circuit, can be adjustedessentially independently.

Therefore, it is possible to perform operations such as intentionallysetting the inductance of the auxiliary coil (Lw) to be smaller than theinductance of the main coil (Lx) and still achieving good resonantoperation. This measure of the invention results in that even under theconditions when the output voltage changes greatly, as in the case inwhich the high pressure discharge lamp is used as a load, basicoperation of the DC-DC converter part of the voltage reduction-buck typeis fixed, depending largely only on the inductance of the main coil (Lx)when the resonant capacitor has a large enough capacitance.

On the other hand, since the auxiliary coil (Lw) is located in the paththrough which energy is supplied to the basic DC-DC converter part ofthe voltage reduction-buck type, the magnetic energy stored in theauxiliary coil (Lw) during the interval of the ON state of the mainswitching device (Qx) is essentially proportional to the energy suppliedto the load during each period of switching operation. This relationhardly depends on the voltage applied to the load.

Therefore, when the wattage supplied to the load does not changegreatly, the voltage charged in the resonant capacitor (Cw) does notchange greatly under the conditions under which the output voltagechanges greatly. The resonance phenomenon in the main coil (Lw), whichhas an intentionally smaller inductance than the main coil (Lx),therefore becomes difficult to be influenced by the fluctuation ofconditions for the load. This feature is one of the major advantages ofthe invention.

At the time (t3), which is shown in FIG. 2, the resonant voltage of theresonant capacitor (Cw) reaches a peak value, and the resonant currentflowing in the auxiliary coil (Lw) reaches 0 and then begins to flow inthe direction opposite the previous direction. As was described above,it becomes apparent that zero voltage switching is achieved when thetransition of the above described auxiliary switching device (Qw) intothe ON state is completed within an interval (τx) which begins at thetime (t2), which is prior to the time (t3) and at which the mainswitching device (Qx) is shifted into the OFF state. Furthermore, duringthe interval, current is flowing in the antiparallel diode (Dqw) andonly the forward voltage of the antiparallel diode (Dqw) is formed forthe voltage of the auxiliary switching device (Qw).

This means that by setting the timing of shifting the auxiliaryswitching device (Qw) into the ON state to be shorter than the length oftime of τxx of the interval (τx) such that enough time is ensured toprevent this timing from coinciding with the ON interval of the mainswitching device (Qx), the switching loss can also be kept low duringthe switching operation of the auxiliary switching device (Qw). This isone of the major advantages of the invention.

As was described above, generally, control is exercised such that,before the main switching device (Qx) reaches the ON state at the time(t4) shown in FIG. 2, the auxiliary switching device (Qw) is shiftedinto the OFF state by a switch closing prohibition interval (τy).

As was described above, at the time (t2), within the interval duringwhich the forward current is flowing in the main switching device (Qx),the main switching device (Qx) is shifted into the OFF state, and thecurrent of the auxiliary coil (Lw) is continued. In this way, currentflows in the closed loop comprised of the auxiliary coil (Lw), the abovedescribed resonant capacitor (Cw) and the antiparallel diode (Dqw) ofthe auxiliary switching device (Qw). In the same way, this time, in theline path which passes through the region in which the main switchingdevice (Qx) is present and located outside of the closed loop, currentbegins to flow in such a way that the current of the auxiliary coil (Lw)is continued when the auxiliary switching device (Qw) reaches the OFFstate at the time (t4).

However, in this case, since the direction of the current of theauxiliary coil (Lw) which continues to try to flow is opposite that atthe time (t2), the current also begins to flow in the line path whichpasses through the area in which the main switching device (Qx) ispresent in the opposite direction, i.e., in the direction in which thebackward current flows in the main switching device (Qx). This meansthat current begins to flow via the line path consisting of the abovedescribed auxiliary coil (Lw), the fly-wheel diode (Dx) and theantiparallel diode (Dqx) which is connected parallel to the mainswitching device (Qx), from the grounding terminal of the DC source(Vin) to the positive terminal.

Here, the electrical charge which is charged in the parasiticelectrostatic capacitance of the main switching device (Qx) iswithdrawn. Afterwards, during the interval in which in the antiparalleldiode (Dqx) current is flowing, a state is maintained on the two ends ofthe main switching device (Qx) in which only a forward voltage of theantiparallel diode (Dqx) is formed.

The antiparallel diode (Dqw) is present as an outside element, forexample, in the case in which the auxiliary switching device (Qw) is aMOSFET. It can also be used as such.

The phenomenon that, by operation at time (t4), current flows from thegrounding terminal of the DC source (Vin) to the positive terminal meansthat the energy of resonance operation which has been stored in theauxiliary coil (Lw) is regenerated in the DC source (Vin) and one of themajor advantages of the invention is that energy is not wasted.

As was described above, the main switching device (Qx) is shifted intothe ON state at the time (t5) shown in FIG. 2 after expiration of theswitch closing prohibition interval (τy), which starts when theauxiliary switching device (Qw) reaches the OFF state. This is completedwithin an interval with a state in which in the antiparallel diode (Dqx)current is flowing and in which on the two ends of the main switchingdevice (Qx) only the forward voltage of the antiparallel diode (Dqx)forms.

By this measure, the current flowing in the antiparallel diode (Dqx)finally reaches 0 at the time (t6) shown in FIG. 2. Zero voltageswitching can be achieved when the current is next inverted and flows inthe forward direction of the main switching device (Qx). This meansthat, in the main switching device's (Qx's) transition into the ONstate, the switching loss can be kept low and the advantage of theinvention can be exploited.

FIG. 2 shows the interval (τz) from the time (t4) at which the auxiliaryswitching device (Qw) is shifted into the OFF state until the time (t6)at which the current flowing in the antiparallel diode (Dqx) reaches 0.The interval is depicted in the figure as relatively long for the easeof drawing. In actual switching operation, the interval (τz) is a shortinterval since the parasitic electrostatic capacitance of the mainswitching device (Qx) is normally a few pF to a few dozen pF, thereforeis small.

During the switch closing prohibition interval (τy), it is necessary toset the timing by which the auxiliary switching device (Qx) is shiftedinto the ON state to be shorter than the length of time of TZZ of theinterval (τz) such that enough time is ensured to prevent this timingfrom coinciding with the ON interval of the above described mainswitching device (Qx). When this condition is satisfied, the switchclosing prohibition interval (τy) can be set to be constant or can bechanged depending on conditions.

As was described above, according to the first aspect of the invention,the switching loss can be reduced in the transition operation of themain switching device (Qx) into the ON state. The auxiliary coil (Lw) isarranged independently of the circuit arrangement of the basic DC-DCconverter of the voltage reduction-buck type. Because the inductance ofthe auxiliary coil (Lw) is intentionally set to be smaller than theinductance of the main coil (Lx), and because the resonant capacitor isset to be great, the resonance phenomenon for the auxiliary coil (Lw)becomes less susceptible to the fluctuation of conditions at the load.Therefore, in a wide, variable range of the continuity ratio of the mainswitching device, the switching loss can be reduced.

Furthermore, if the parameters of the auxiliary coil (Lw) and of theresonant capacitor (Cw) are set in a suitable manner hereby, theswitching loss can be reduced in transition operation of the auxiliaryswitching device (Qw) into the ON state. Also, the energy of theresonant operation of the auxiliary coil (Lw) in the DC source (Vin) canbe regenerated. Therefore, as a whole a DC-DC converter with highefficiency can be built.

FIG. 14 shows for information purposes the waveforms of the mainwaveforms measured in reality for the circuit shown above in FIG. 1. Theswitching devices and the parameters in this circuit are shown below.

-   -   auxiliary coil (Lw): 35 μH    -   resonant capacitor (Cw): 1 μF    -   main switching device (Qx): 2 SK 2843 (Toshiba)    -   auxiliary switching device (Qw): 2 SK 2843 (Toshiba)    -   main coil (Lx): 2.2 mH    -   fly-wheel diode (Dx): YG 1912S6 (Fujidenki)    -   smoothing capacitor (Cx): 0.47 μF    -   switching frequency: 100 kHz    -   load (Zx): 30 Ω    -   input voltage: 370 V    -   output wattage: 150 W    -   output voltage: 67 V    -   output current: 2.24 A

The advantage of one development of the first aspect of the invention isdescribed below.

As is discussed above, in the circuit arrangement described above (FIG.1), because the main switching device-gate driver signal (VxG) fordriving the above described main switching device (Qx) and the auxiliaryswitching device-gate driver signal (VwG) for driving the auxiliaryswitching device (Qw) are activated in alternation, i.e., in aninverting manner, resonant operation is carried out.

Since, as is described above, current supply from the DC source (Vin) tothe load side is carried out via the resonant capacitor (Cw), in thecase of a small difference between the voltage of the DC source (Vin)and the output voltage of the DC-DC converter of the voltagereduction-buck type, the output current serviceability is reduced. Forexample, under these conditions, in the case in which amplification ofthe output current serviceability as the DC-DC converter becomesadvantageous, it is necessary to switch the circuit arrangement suchthat current feed to the load side is not carried out via the resonantcapacitor (Cw).

In the circuit arrangement of the invention the auxiliary switchingdevice (Qw) is connected in parallel to the LC series connection inwhich the auxiliary coil (Lw) and the resonant capacitor (Cw) areconnected in series. Therefore, by controlling the auxiliary switchingdevice (Qw), resonant operation can be stopped easily by shifting theauxiliary switching device (Qw) into the ON state when the ON state ofthe main switching device (Qx) is reached.

FIG. 3 shows the situation for implementation of this by driving themain switching device (Qx) and the auxiliary switching device (Qw)in-phase in the circuit described above using FIG. 1. FIG. 3 shows theinterval (Ton) during which the main switching device (Qx) is shiftedinto the ON state. The main switching device-gate driver signal (VxG)and the auxiliary switching device-gate driver signal (VwG) are operatedso that the interval (Ton) is encompassed by an interval (Tw1) duringwhich the auxiliary switching device (Qw) is shifted into the ON state.But a transition can also be carried out with the same timing.

FIG. 4 shows the situation in which resonant operation is likewisestopped by steady-state shifting of the auxiliary switching device (Qw)into the ON state.

Operation is simpler for the control shown in FIG. 4. However, theauxiliary switching device-gate driver signal (VwG) cannot be madedirect-current, as in the case in which, for example, driving of theauxiliary switching device (Qw) using a pulse transformer is carriedout. In the case in which the auxiliary switching device (Qw) cannot beshifted into the ON state in a steady-state manner, it is thereforeadvantageous to exercise control in the manner described above usingFIG. 3.

With respect to the division of conditions for switching a state inwhich resonant operation is carried out, i.e., a resonance mode, and astate in which the resonant operation is stopped, i.e., a resonance stopmode, into one another, as was described above, it is possible toproceed as follows:

-   -   For example, as was described above, if the difference between        the voltage of the DC source (Vin) and the output voltage of the        DC-DC converter is small, the resonance stop mode is started.    -   In the other case there is a circuit by which for example the        continuity ratio of the main switching device (Qx) or an amount        which correlates to the continuity ratio is monitored for        control to achieve the resonance mode. Control can be exercised        in such a way that the resonance stop mode is started when it        has been determined that this continuity ratio is greater than        or equal to a predetermined value. Or there can simply be a        circuit by which the output voltage of the DC-DC converter is        monitored and control can be exercised such that the resonance        stop mode starts when it has been determined that this voltage        is greater than or equal to a predetermined value.

Furthermore it is also possible to proceed as follows:

Based on the property of the device to be used, as in the case of anapplication for the device described below for operating a dischargelamp, switching of the resonance mode and the resonance stop modeagainst one another can be carried out simply by time control based onthe operation sequence when it is known, for example, beforehand that itis advantageous for there to be the resonance stop mode within a certaininterval at the beginning.

The advantage of a second aspect of the invention is described below. Asdescribed in the prior art, the discharge voltage of a high pressuredischarge lamp changes greatly and also vigorously depending on thedischarge state, i.e., the state in which a no-load voltage is applied(state before the start of discharge), the glow discharge state, thestate of a transient arc discharge, and the state of a steady-state arcdischarge. Therefore, the converter for supply of a high pressuredischarge lamp is required to have the property that enables a promptchange of the continuity ratio according to the discharge voltage of thehigh pressure discharge lamp in a wide, variable range with PWM control.Furthermore, there is a demand for a property that enables maintenanceof operation in which the switching loss is reduced by resonanceoperation.

As described above, there is an auxiliary coil (Lw) of the circuitarrangement of the underlying DC-DC converter of the voltagereduction-buck type. Because the inductance of the auxiliary coil (Lw)is set intentionally smaller than the inductance of the main coil (Lx),the resonance phenomenon for the auxiliary coil (Lw) becomes lesssusceptible to the fluctuation of conditions at the load. Therefore, ina wide, variable range of the continuity ratio of the main switchingdevice the switching loss can be reduced. It is therefore suited as aconverter for supply of a high pressure discharge lamp. A devicearranged using it therefore works advantageously for operating a highpressure discharge lamp.

FIG. 5 shows the circuit arrangement of a device for operating a highpressure discharge lamp (Ld) in a simplified representation in which theDC-DC converter for supplying a high pressure discharge lamp is theDC-DC converter of the voltage reduction-buck type of the invention,which was described above using FIG. 1.

To obtain a device for operating a high pressure discharge lamp (Ld), inaddition to FIG. 1, there are a starter (Ui), a shunt resistor (R1) asthe output current detector, voltage divider resistors (R2, R3) as theoutput voltage detectors and a feedback control element (Fb).

In the starter (Ui), a capacitor (Ci) is charged via a resistor (Ri) bya lamp voltage (VL). When a gate driver circuit (Gi) is activated, byclosing the switching device (Qi) is comprised of a thyristor or thelike, the capacitor (Ci) is discharged by the primary winding (Pi) ofthe transformer (Ti), by which in the secondary winding (Hi) a highvoltage pulse is formed and applied between the electrodes (E1, E2) ofthe two poles of the high pressure discharge lamp (Ld). In this way,within the discharge space (Sd) an insulation breakdown occurs and thedischarge of the high pressure discharge lamp (Ld) begins.

A lamp current determination signal (Sxi) is input by the shunt resistor(R1) and lamp voltage determination signals (Sxv) are input by thevoltage divider resistors (R2, R3) to the feedback control element (Fb)from which a PWM signal (Sa) is sent to the driver control element (Gw).The driver control element (Gw) carries out drive control of the mainswitching device (Qx) and the auxiliary switching device (Qw) in thisway.

The feedback control element (Fb) based on the lamp voltagedetermination signal (Sxv) before the start of discharge of the highpressure discharge lamp (Ld) carries out feedback control of the no-loadvoltage. That the starter (Ui) produces a high voltage pulse and thatthe discharge of the high pressure discharge lamp (Ld) has begun can bedetermined by the feedback control element (Fb), for example, by thelamp current determination signal (Sxi).

Furthermore the feedback control element (Fb) carries out the following:

-   -   The lamp wattage setpoint is divided by the lamp voltage value        which is computed by the lamp voltage determination signal        (Sxv);    -   In this way, the lamp current setpoint is computed at this        instant;    -   A lamp current setpoint signal that corresponds to this lamp        current setpoint is generated internally; and    -   Feedback control of the lamp current is carried out such that        the difference between it and the lamp current determination        signal (Sxi) is reduced.

However, as described above, since immediately after the transition intoa transient arc discharge via a glow discharge, the lamp voltage is lowand since the lamp current setpoint computed according to this lampvoltage value becomes unduly large, it is advantageous to exercisecontrol so that the lamp current value is kept at the upper boundaryvalue until finally the lamp voltage increases and until an appropriatelamp current setpoint is computed.

FIG. 6 shows one embodiment according to the first aspect of theinvention. A version of a DC-DC converter of the invention is shown inwhich the auxiliary coil (Lw) and the resonant capacitor (Cw) arelocated downstream of the main switching device (Qx). Here the sameaction as in FIG. 1 can be used.

In the circuit arrangement shown in FIG. 6, a diode (Dw) is connected inparallel to the series connection of the auxiliary coil (Lw) and themain switching device (Qx) in order to avoid the following case.

There is specifically a case in which relatively great ringing arises atthe electrical potential of the node of the nodal point between the mainswitching device (Qx) and the main coil (Lx) in the transition of themain switching device (Qx) into the ON state. When due to the presenceof this ringing neither the disadvantage that for example the ratedvalues of the switching devices are exceeded nor a similar disadvantageoccurs, the diode (Dw) can also be omitted.

FIG. 7 shows one embodiment of the first aspect of the invention.

Here, an embodiment of a DC-DC converter of the invention is shown, inwhich the auxiliary coil (Lw) is located on a line (ground line) of theDC source (Vin) which is opposite the line on which the main switchingdevice (Qx) and the main coil (Lx) are located next to one another. Herethe same action as in FIG. 1 can be used.

FIG. 8 shows the arrangement of the driver control element (Gw) and thefeedback control element (Fb) of a DC-DC converter of the invention in asimplified representation.

The feedback control element (Fb) comprises the following:

-   -   an arithmetic circuit (Uj) which computes the lamp current        setpoint by dividing the lamp wattage setpoint by a lamp voltage        value which is computed on the basis of lamp voltage        determination signal (Sxv),    -   a driving capacity control circuit (Ud) which carries out pulse        width modulation with feedback such that the difference between        the lamp current setpoint signal (Sbv) which has been computed        by this arithmetic circuit (Uj) and the lamp current        determination signal (Sxi) is reduced at this time; and    -   a resonance control circuit (Uc) which generates a resonance        prohibition signal (Sc) which is used to switch the resonance        mode and the resonance stop mode into one another and which        prohibits resonant operation.

The PWM signal (Sa) is output by the driving capacity control circuit(Ud).

Here, if the resonance prohibition signal (Sc) is inactive, the mainswitching device (Qx) and the auxiliary switching device (Qw) must beshifted in alternation into the ON state. Therefore the main switchingPWM signal (Sax) which is to become a driver signal of the mainswitching device (Qx) and the auxiliary switching PWM signal (Saw) whichis to become an inversion signal of it, i.e., a driver signal of theauxiliary switching device (Qw), are generated. If conversely theresonance prohibition signal (Sc) is activated, the auxiliary switchingPWM signal (Saw) is generated as an in-phase signal with respect to themain switching PWM signal (Sax) or as a signal which shifts theauxiliary switching device (Qw) into the ON state in a steady-statemanner.

This generation of the auxiliary switching PWM signal (Saw) with respectto the main switching PWM signal (Sax) in an inverting, in-phase orsteady-state manner takes place in a signal conversion part (Uf) whenthe resonance prohibition signal (Sc) is received. The generated mainswitching PWM signal (Sax) and the generated auxiliary switching PWMsignal (Saw) are converted at the driver control element (Gw) intosignals which are used to drive the switching devices.

Since control is exercised in such a way that only after the auxiliaryswitching device (Qw) reaches the OFF state is the main switching device(Qx) shifted into the ON state within a given time, this time can beregulated by adding a delay circuit (Un) for delaying the timing fordriving the main switching device (Qx).

Next, there are circuits for driving the main switching device (Qx) andthe auxiliary switching device (Qw), for example driver circuits (Uqx,Uqw) comprised of a pulse transformer, a high-side-driver or the like.In this way, driver signals (Sqx, Sqw) are generated for the respectiveswitching device and the respective switching device is subjected toON-OFF control.

A microprocessor (not shown in the drawings) can be installed in thefeedback control element (Fb) and thus the discharge state of the highpressure discharge lamp can be identified and a relatively complicatedsequence which is subject to normal operation control can be processed.Here, it is advantageous to proceed as follows:

-   -   The lamp voltage determination signal (Sxv) is converted by AD        conversion into a lamp voltage value.

The computation of the lamp current setpoint which satisfies a lampwattage setpoint is done by the microprocessor.

A lamp current setpoint signal is generated by a D/A converter.

FIG. 9 shows an embodiment of part of the feedback control element (Fb)of the invention and of the circuit arrangement of the driver controlelement (Gw) of a DC-DC converter.

In the driving capacity control circuit (Ud), the error of the lampcurrent determination signal (Sxi) at this time is integrated withrespect to the lamp current setpoint signal (Sbv) using an errorintegrator which consists of a capacitor (Cp) and an operationalamplifier (Ade). The integrated integration signal (Sd1) is comparedusing a comparator (Cmg) to a sawtooth wave which has been produced inan oscillator for producing sawtooth waves (Osc). In this way the PWMsignal (Sa) is generated such that a signal is obtained for which themagnitude of the continuity ratio changes according to the magnitude ofthe integration signal (Sd1), i.e., a driver signal is obtained which issubjected to PWM control for the main switching device (Qx).

In FIG. 9, for the resonance control circuit (Uc) the magnitude of theintegration signal (Sd1) is compared to the signal of a referencevoltage signal generator (Vtc) using a comparator (Cmc), thus it isassessed whether a state is present or not in which the integrationsignal (Sd1) produces a continuity ratio which is greater than a givenvalue, and the resonance prohibition signal (Sc) is generated such thatwhen the integration signal (Sd1) produces a continuity ratio which isgreater than a given value, the resonance prohibition mode is enteredand that in the other case the resonance mode is entered.

FIG. 9 describes a case in which the gate signal of the auxiliaryswitching device (Qw) is generated using the pulse transformer (Tw).When the resonance prohibition signal (Sc) of the resonance prohibitionmode is chosen, the auxiliary switching device (Qw) is therefore notshifted into the ON state in a steady-state manner, but is drivenin-phase with the main switching device (Qx).

In the resonance mode, with the gate signal of the auxiliary switchingdevice (Qw), the main switching device (Qx) and the auxiliary switchingdevice (Qw) are shifted alternately into the ON state. Therefore, thePWM signal (Sa) and an inversion signal of it are required. Conversely,in the resonance prohibition mode a signal for which the PWM signal (Sa)has been inverted is not necessary.

Therefore, there is a buffer (Bx) which with respect to the abovedescribed PWM signal (Sa) produces the above described in-phase mainswitching PWM signal (Sax) (noninverting, installed if necessary).Furthermore there is an inverter (Bw) which generates an inversionsignal of it. Using a data switch (Se1) and based on the above describedresonance prohibition signal (Sc), a signal is chosen and sent to therear stage as the above described auxiliary switching PWM signal (Saw)for producing the gate signal of the above described auxiliary switchingdevice (Qw).

In doing so, the main switching PWM signal (Sax) is output via a buffer(Bfx) to the next stage, since a delay circuit is formed which followsthe time constant of a CR circuit consisting of a resistor (Rx2) and acapacitor (Cx1). In this delay circuit, a delay can be taken to asufficient extent in the case of reaching “High”. Conversely, in thecase in which the voltage of the buffer (Bfx) drops from “High” to“Low”, control is exercised in such a way that parallel to the resistor(Rx2) a diode (Dx1) is added, an electrical charge is quickly withdrawnfrom the capacitor (Cx1) and thus the delay time is shortened. Thus,only the signal of when the main switching device (Qx) is turned on isdelayed.

Then, the signal which has been output from the buffer (Bfx) istransmitted via a base resistor (Rx3) to a driver circuit (Uqx) fordriving the main switching device (Qx). From the nodal point between thedriver circuit (Uqx) and the switching devices (Qx2, Qx3), a signal istransmitted to the primary winding (Px) of the pulse transformer (Tx)via a capacitor (Cx2) and a resistor (Rx4) as the current limitationresistor. A resistor (Rx5) which is to become the gate resistor of themain switching device (Qx) is connected from the secondary winding (Sx)of the pulse transformer (Tx). A resistor (Rx6) is connected thereto andis connected between the drain and the source electrode in order tosmoothly turn off the main switching device (Qx). These signals (Sqx1,Sqx2) are transmitted to the main switching device (Qx).

On the other hand, a delay is added to the auxiliary switching PWMsignal (Saw) by a delay circuit (Um) which likewise consists of aresistor (Rw2), a capacitor (Cw1), a diode (Dw1) and a buffer (Bfw). Thesignal which has been output via the buffer (Bfw) is transmitted to theswitching devices (Qw2, Qw3) via the base resistor (Rw4) and proceedsfrom the nodal point between the switching devices (Qw2, Qw3) via acapacitor (Cw2) and a resistor (Rw7) as a current limitation resistor tothe primary winding (Pw) of the pulse transformer (Tw). A gate resistor(Rw5) of the auxiliary switching device (Qw) and a resistor (Rw6) whichis connected between the drain and the source electrode is connected tothe secondary winding (Sw) for smoothly turning off the auxiliaryswitching device (Qw). Generated driver signals (Sqw1, Sqw2) aretransmitted to the auxiliary switching device (Qw).

By this arrangement, in the control circuit shown in FIG. 9, the deviceof the invention for operating a high pressure discharge lamp can becontrolled with feedback such that the error between the lamp currentdetermination signal (Sxi) and the lamp current setpoint signal (Sbv)decreases. Under the condition under which the continuity ratio of themain switching device (Qx) is smaller than a given value, i.e., underwhich the output voltage is relatively low, a resonance mode can beachieved and the main switching device (Qx) and the auxiliary switchingdevice (Qw) can be subjected to ON-OFF control in such a way that theswitching loss is reduced. Conversely, under the condition under whichthe continuity ratio of the main switching device (Qx) is greater than agiven value, i.e., under which the output voltage is relatively high, aresonance stop mode is obtained and the same output currentserviceability as in a conventional DC-DC converter of the voltagereduction-buck type is ensured.

In the resonance mode for the main switching PWM signal (Sax) and theauxiliary switching PWM signal (Saw) for the two switching devices,specifically for the main switching device (Qx) and the auxiliaryswitching device (Qw), there are delay circuits (Un, Um). In this way,the switching devices are prevented from being turned on at the sametime.

In this embodiment, the integration signal (Sd1) is monitored as anamount correlating to the continuity ratio of the main switching device(Qx). Control is exercised such that in the case of a large continuityratio the resonance stop mode starts. This control process is very wellsuited as a DC-DC converter for a device for operating a discharge lamp.

The reason for this is the following:

As was described above, in the state in which a no-load voltage with ahigh output voltage is applied, no output current flows. In a glowdischarge likewise with a high output voltage the output current is farsmaller than in an arc discharge. Under the condition under which theseoutput voltages are high, the wattage supplied to the load is thereforeoriginally small. The loss in the main switching device (Qx) istherefore low. Therefore an increase of the loss by the starting of theresonance stop mode can be ignored for the most part.

For example, TL494 from Texas Instruments or the like can be used as acommercial IC in which functional units such as the operationalamplifier (Ade) described above using FIG. 9, the oscillator shown inFIG. 9 for producing sawtooth waves (Osc), the comparator (Cmg) shown inFIG. 9 for comparison with the sawtooth waves and the like areintegrated.

FIG. 10 shows an embodiment according to the second aspect of theinvention. This embodiment is a device for operating a high pressuredischarge lamp using a starter which is called an external trigger type.In the high pressure discharge lamp (Ld), there is an auxiliaryelectrode (Et) besides the electrodes for the main discharge such thatit does not come into contact with the discharge space (Sd). Betweenthis auxiliary electrode (Et) and the first and second electrodes, ahigh voltage is applied, by which plasmas are produced in the dischargespace (Sd). The main discharge is started by a voltage (no-load voltage)that is applied beforehand between the first electrode and the secondelectrode, with the plasmas acting as the initiating substance.

FIG. 11 shows an embodiment according to the second aspect of theinvention. Here, a device for operating a high pressure discharge lampof the external trigger type is shown in which an AC voltage is appliedto the high pressure discharge lamp (Ld).

It is possible to apply an alternating discharge voltage to the highpressure discharge lamp (Ld) by installing a full bridge inverter. Thefull bridge inverter is obtained by adding switching devices to the DCoutput part of the DC-DC converter. The added switching devices aredriven by a control circuit part (Gf) for full bridge driving and arecontrolled in such a way that the diagonal elements are driven inalternation so that the switching devices (Q1, Q4) (Q2, Q3) are closedat the same time as the diagonal elements of the full bridge inverter.

FIG. 12 shows a schematic of the state of the lamp voltage (VL), of thelamp current (IL) and of the resonance prohibition signal (Sc) accordingto one embodiment of the second aspect of the invention.

An embodiment was described above in which the integration signal (Sd1)is monitored as the amount which correlates to the continuity ratio ofthe main switching device (Qx) and in which control is exercised suchthat in the case of a large continuity ratio the resonance stop mode isstarted. Subsequently, one embodiment in an even more simplifiedrepresentation is shown in which the resonance stop mode and afterwardsthe resonance mode begin from the starting of the discharge lamp until agiven time expires.

In FIG. 12, during the interval (τv) a no-load voltage (Vs) is output.FIG. 12 shows a situation in which at the time (τg) the starter isoperated, in which during an interval (τg) a glow discharge with acertain glow discharge voltage (Vg) forms, and in which afterwards anarc discharge is formed. However, in fact, there is also a case in whicha transition to an arc discharge takes place once, in which a return toa glow discharge occurs, in which in the case of a transition again toan arc discharge the extinction of the discharge takes place when theglow discharge returns and in which proceeding from the output of theno-load voltage a repeated attempt is made. After expiration of theinterval during which a transition phenomenon such as a glow discharge,extinction of the discharge and the like, occurs, the transition intothe arc discharge is completed.

Since the lamp voltage is low during the interval (τe) immediately aftercompletion of the transition into the arc discharge, the lamp currentbecomes unduly large when an attempt is made to supply the rated wattageto the lamp during this interval. The lamp current is limited to asuitable current value of the upper limit (Ic) and output is carriedout. During this interval (re) therefore a wattage which is lower thanthe rated wattage is applied to the lamp.

The interval during which the transition phenomenon occurs is restrictedto a limited time. Expressed in greater detail, such a transitionphenomenon is generally continued when the interval during which thelamp is off is short after completion of lamp operation when the lamptemperature is too high. This repeated induction of such a transitionphenomenon shortens the lamp service life. When the transition into thearc discharge is not completed within a given interval, for purposes oflamp protection the attempt to operate the lamp is stopped. As a resultthe interval during which the transition phenomenon occurs is restrictedto a limited time. When the lamp temperature is low enough, thetransition into the arc discharge is of course in fact completed withinthe given interval.

As described above, under the condition under which a high voltage isoutput, as in a glow discharge, the resonance stop mode can be started.As discussed, the interval during which the resonance stop mode is to bestarted is restricted to a limited time. It is therefore relativelypractical to exercise control in the following manner.

A suitable interval is established without the confirmation of whetherin fact a glow discharge takes place or not. Within this interval, theresonance stop mode is started after initiating the start-up of the lampand the resonance mode is started after this interval expires.

A case was described above using FIG. 12 in which within an interval(τr) after the beginning of start-up of the lamp the resonanceprohibition signal (Sc) is activated, in which the resonance stop modeis started, and in which the resonance mode is started after thisinterval expires. However, in FIG. 12, a polarity is assumed for whichthe resonance stop mode is started when the resonance prohibition signal(Sc) is at a high level.

The interval (πr) will be fixed in the following manner:

It should be greater than the interval during which a transitionphenomenon such as the glow discharge, the extinction of the dischargeand the like, can occur. But it should be shortened to the extent thatit does not exceed the end of the interval (πe) during which a wattagelower than the rated wattage is supplied to the lamp.

FIG. 13 shows one embodiment of part of a feedback control element (Fb)of the invention and the circuit arrangement of the driver controlelement (Gw) of a DC-DC converter. In this arrangement the auxiliaryswitching device (Qw) is shifted into the ON state when the resonanceprohibition signal (Sc) is at a low level.

The delay circuits (Un, Um) and the driver circuit (Uqx) were describedabove using the embodiment described in FIG. 9.

For the gate signal of the auxiliary switching device (Qw), the mainswitching device (Qx) and the auxiliary switching device (Qw) areshifted in alternation into the ON state. Therefore the PWM signal (Sa)and an inversion signal thereof are needed. Therefore, with respect tothe PWM signal (Sa) there are two switching devices (Qx1, Qw1). Theswitching device (Qx1) is an emitter follower by a resistor (Rx1) andgenerates a main switching PWM signal (Sax) which is an in-phase signallike the PWM signal (Sa). A resistor (Rw1) is connected to the switchingdevice (Qw1). Furthermore the switching device (Qw1) is an emitterground and generates an auxiliary switching PWM signal (Saw) which is aninversion signal with respect to the PWM signal.

In the signal conversion part (Uf), the auxiliary switching PWM signal(Saw), which will become the driver signal of the auxiliary switchingdevice (Qw), is generated. When the resonance prohibition signal (Sc) isat a low level, the switching device (Qc) is shifted into the ON stateand the switching device (Qw1) is shifted into the OFF state via aresistor (Rw0) which is connected in series to a resistor (Rx0). As aresult the auxiliary switching PWM signal (Saw) reaches a high level,passes through the delay circuit (Um) and is transmitted to the drivercircuit (Uqw).

Since it is difficult using a pulse transformer to shift the auxiliaryswitching device (Qw) into the ON state in a steady-state manner, it isnecessary to use an insulation means such as a high-side-driver, aphotocoupler or the like. If the auxiliary switching device (Qw) is aFET, it is necessary to connect the source terminal to the ground of thedriver control element (Gw).

The driver circuit described above using FIG. 13 is an example in whicha high-side-driver (Hsd) is used which comprises a signal generator(Hsc) and switching devices (QH, QL) that can directly drive theauxiliary switching device (Qw), and the like. The voltage which becomesthe current source of the output of this high-side-driver (Hsd) issubjected to a “charge pump up” by an arrangement of a capacitor (Ch)and a diode (Dh) by the ON-OFF of the main switching device (Qx). In thecapacitor (Ch), a current source for driving the auxiliary switchingdevice (Ow) is produced.

The auxiliary switching PWM signal (Saw) passes through the delaycircuit (Um), passes through the high-side-driver (Hsd) and is outputvia a resistor (Rh). The output here is at a high level. The generateddriver signals (Sqw1, Sqw2) are transmitted to the auxiliary switchingdevice (Qw) with a high voltage. Resonance operation can be easilyswitched by this arrangement by the resonance prohibition signal (Sc).

In the DC-DC converter of the invention zero voltage switching isaccomplished in the transition of the auxiliary switching device (Qw)into the ON state. Therefore the occurrence of noise can also be keptfundamentally low during this switching.

In these application documents, only what is most necessary in thecircuit arrangement is described in order to explain the operation,function and action of the light source device of the invention. It istherefore assumed that the other details of circuit operation which isdescribed in the embodiments, for example, the polarity of the signals,the specific choice, the specification addition and omission of circuitcomponents or concepts such as changes and the like are intensivelycarried out for reasons of facilitating the procurement of componentsand for economic reasons, in the practice of building an actual device.

It is assumed that especially a device for protecting the circuitcomponents of a feed device, such as switching devices such as a FET orthe like, against damage factors such as a wattage which exceeds acertain value, a current which exceeds a certain value, overheating andthe like, or a device which reduces formation of radiation noise andline noise that arise according to operation of the circuit componentsof the feed device, or which prevents the noise formed from beingreleased to the outside, such as a snubber circuit, a varistor, aclamping diode (including the “pulse-by-pulse method”), a currentlimiter circuit, a noise filter reactor coil with a “common mode” or a“normal mode”, a noise filter capacitor and the like if necessary isadded to the respective part of the circuit arrangements which aredescribed in the embodiments.

The invention according to the first aspect can provide a DC-DCconverter by which the disadvantage of a conventional DC-DC converter,i.e., the disadvantage of difficult implementation of reducing theswitching loss in a wide variable range of the continuity ratio of themain switching device with low costs, is eliminated.

The invention according to the second aspect can provide a device foroperating a high pressure discharge lamp by which the disadvantage of aconventional device for operating a high pressure discharge lamp, i.e.,the disadvantage of difficult implementation of reducing the switchingloss with low costs, is eliminated.

1. A DC-DC converter of the voltage reduction-buck type comprising: adirect current source (Vin); an ON-OFF-controllable main switchingdevice (Qx); a main coil (Lx) in series connection to the main switchingdevice (Qx); a fly-wheel diode (Dx) operatively arranged so that theinduction current of the main coil (Lx) flows when the main switchingdevice (Qx) is shifted into the OFF state; a smoothing capacitor (Cx)for smoothing the output of the main coil (Lx); an auxiliary coil (Lw);a resonant capacitor (Cw); and an ON-OFF-controllable auxiliaryswitching device (Qw), wherein the auxiliary coil (Lw) and the resonantcapacitor (Cw) are series-connected to form a LC series connection,wherein the LC series connection, the main switching device (Qx) and thefly-wheel diode (Dx) are series connected, wherein the auxiliaryswitching device (Qw) is connected in parallel to the LC seriesconnection, and wherein the main switching device (Qx) and the auxiliaryswitching device (Qw) are controlled such that they are alternativelyshifted into the ON state, and the main switching device (Qx) is shiftedinto the ON state within a given time after the auxiliary switchingdevice (Qw) has been shifted into the OFF state.
 2. The DC-DC converteras set forth in claim 1, wherein when in the state in which a resonanceprohibition signal (Sc) is activated and when the ON state of the mainswitching device (Qx) is reached, the auxiliary switching device (Qw)also is shifted into the ON state, instead of the main switching device(Qx) and the auxiliary switching device (Qw) being controlled such thatthey are alternatively shifted into the ON state and the main switchingdevice (Qx) being shifted into the ON state within a given time afterthe auxiliary switching device (Qw) has been shifted into the OFF state.3. A device for operating a high pressure discharge lamp (Ld) in which adischarge space (Sd) is filled with a discharge medium and an opposedpair of electrodes (E1, E2) for the main discharge are positioned withinthe discharge space, the device comprising the DC-DC converter ofclaim
 1. 4. A device for operating a high pressure discharge lamp (Ld)in which a discharge space (Sd) is filled with a discharge medium and anopposed pair of electrodes (E1, E2) for the main discharge arepositioned within the discharge space, the device comprising the DC-DCconverter of claim 2.